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Concave and convex mirrors
0 If you look at your reflection in a spoon, your image
might appear inverted (upside down). The attitude of
an image describes whether the image is upright or
upside down in relation to the object.
Concave and convex mirrors
0 Concave mirrors are curved like the inside of a bowl.
These types of mirrors are often used in astronomers’
telescopes.
0 Convex mirrors curve outward, like a balloon.
Images formed by curved
mirrors
0 A concave mirror can produce two types of images. If
an obkect is close to the mirror, the image seen is
upright and larger than the real object.
0 On the other hand, when the object is far from the
concave mirror, the image is inverted and smaller than
the real object.
Images formed by curved
mirrors
0 Convex mirrors are more common than concave
mirrors. Convex mirrors always produce small,
upright images.
0 Because the image of each object is smaller in a
convex mirror, you see more of the overall scene.
Stop and think
0 Where have you seen convex mirrors before? Why are
they used?
0 Can you think of any uses for concave mirrors?
Lenses and vision
0 A lens is a curved piece of transparent material, such
as glass or plastic. Light refracts as it passes through a
lens, causing the rays to bend.
0 A concave lens is thinner and flatter in the middle
than around the edges. Light passing through the
thicker, more curved areas of the lens will bend more
than light passing through flatter areas. This causes
rays of light to spread out, or diverge, after passing
through the lens.
Lenses and vision
0 A convex lens is thicker in the middle than around the
edges. This causes the light to come together, or
converge.
Lenses and images
0 Lenses are probably the most useful and important of
all optical devices.
0 Eyeglasses were made from lenses are early as the
13th century.
0 The type of lens used in an optical device determines
whether the image is real or virtual.
Lenses and images
0 An image forms where light rays from an object
converge.
0 The light rays spread out from points on the object. A
convex lens refracts these rays so that they come back
together.
Lenses and images
0 However, the lens directs light from the left portion of
the object to the right portion of the image.
0 Similarly, light from the top of the object is directed to
the bottom of the image. Thus, an image formed by a
convex lens is sometimes inverted rather than
upright.
Lenses and images
0 Because your eyes sees the image of the flower by
light rays that are actually coming from the image, the
image is a real image.
0 An image on a movie screen is a real image since light
actually travels from the screen to your eye.
0 Any image that can form on a screen is a real image.
Both film projectors and overhead projectors used a
convex lens to create real images.
Lenses and images
0 Sometimes, light rays only seem to come from the
image.
0 Light rays are coming from the object as if they had
started from behind the mirror. In fact, rays are just
bounding off the mirror, so a virtual image is
produced.
Lenses and images
0 Light rays do not actually come from a virtual image.
0 Plane mirrors always produce virtual images.
Remember that if you cannot form an image on a
screen, the image must be virtual.
0 Sometimes a convex lens can be used to product a
virtual image, like when you use a magnifying glass.
Lenses in humans
0 Your eyes have convex lenses. This lens takes light
rays that are spreading out from an object and, by
refraction, focuses them, or brings them back to a
point.
0 This focusing of light rays allows us to see objects,
0 In a normal eye, light refracts through the lens onto a
light-sensitive area at the back of the eye called the
retina. The image you see is formed on the retina.
Normal vision
Lenses in humans
0 For some people, the eye has a longer shape. This
means the image forms in front of the retina. These
people are near-sighted. They have trouble seeing
distant objects.
0 A concave lens placed in front of the eye helps bend
the light rays for that the image appears on the retina.
Near-sightedness
Lenses in humans
0 For other people, the eye has a shorter shape and the
image has not formed by the time the light reaches the
retina.
0 These people are far-sighted. They have trouble
seeing objects that are close to them.
0 A convex lens helps far-sighted people.
Far-sightedness
Comparing the eye and the
camera
0 There are many similarities between the human eye
and the camera.
0 When you see objects, light is reflected to your eye,
refracted through your lens, and focused on your
retina.
0 In a camera, the lens refracts the light and the film
senses the light.
Stop and think:
0 The word “optics” has not yet been defined, although
nearly everything we’ve discussed in this unit is
related to optics. Think about what you have been
learning, and write your own definition for optics.
Putting it in focus
0 In a camera, if an object moves closer to the film, the lens must
move farther away from the film to keep the image in focus. This
is what you do when you focus the camera.
0 In the human eye, you cannot more the lens farther away from
the retina. Instead, the ciliary muscles change the shape of the
lens.
0 If the object you are looking at comes closer to you, these
muscles make the lens bulge in the middle. This keeps the object
in focus on the retina without having to stretch the eyeball.
Putting it in focus
0 The process of changing the shape of the lens to
adjust for different object distances is called
accommodation.
0 As people become older, the lens becomes less flexible
and loses its ability to change shape. It can no longer
focus on close objects. As a result, many older people
wear convex lenses as reading glasses.
Putting it in focus
0 The shortest distance at which an object is in focus is
called the near point of the eye. The longest distance
is called the far point.
0 For the average adult human eye, the near point is
about 25 cm away. However, babies can focus on
objects only 7 cm away.
0 The far point is said to be infinity. Most people with
normal vision are able to see the stars, and they can
be many thousand of light years away.
Bringing in the light
0 If you are photographing a scene and a cloud suddenly
covers the Sun, the brightness of the scene decreases.
As a result, the amount of light reaching the film
decreases.
0 If the light is too dim, the film will not receive enough
light to record the image clearly.
0 In this case, the camera’s diaphragm and shutter can
be adjusted to allow the correct amount of light to
reach the film.
Bringing in the light
0 The diaphragm is a device that controls the aperture
(the opening) of a lens or optical system.
0 The shutter is a device that limits the passage of light.
The aperture in the diaphragm can let more light into
the camera by being opened wider.
0 The shutter can let in more light by staying open
longer.
Bringing in the light
0 In the human eye, the iris, which is the coloured ring,
functions like the diaphragm of a camera.
0 If the light is dim, the iris increases the size of the
eye’s opening to let in more light.
0 This opening is called the pupil, which appears as
dark as the center of your eye.
Bringing in the light
0 The natural adjustment in the size of the pupils is
called the iris reflex. You are generally unaware of this
reflex, which is extremely rapid.
0 Think about what happens when you walk into a
movie theater mid-afternoon. For the first few
seconds, you probably have difficulty seeing. `
0 The split-second iris reflex is the first in a series of
adjustments your eyes make to enable you to see in
the dark movie theater.
Entrance slip
0 Write down 3 things you’ve learned so far in this unit.
0 Write down 2 new vocabulary words you’ve learned.
0 Write down 1 question you have about optics.
Be prepared to share your answers .
Seeing the image
0 At the back of an old camera, you’ll find the film. The
image is focused on this light-sensitive material. The
light energy causes chemical changes in the film to
record the image.
0 In the human eye, the retina senses the light. When
the cells in the retina detect light, they create small
electrical impulses that travel from the retina to the
brain through the optic nerve.
Seeing the image
0 The point where the optic nerve enters the retina
does not have any light-sensing cells. This point is
called the blind spot.
0 The parts of a camera are contained in a rigid, light
proof box. In the eyes, layers of tissue hold the
different parts together. In order to keep the eye from
collapsing, the eyeball is filled with fluids called
humours. In addition to keeping the eye rigid,
humours help refract the light that enters the eye.
Stop and think:
0 As an object comes closer to a convex lens, what
happens to:
0 The size of the image?
0 The attitude of the image?
Mythbusters
0 http://dsc.discovery.com/tv-
shows/mythbusters/videos/death-ray-minimyth.htm
Extending our vision
0 Human knowledge about our planet and the universe
was very limited until we developed products to
extend our vision, like telescopes, binoculars, and
mircoscopes.
Telescopes
0 Telescopes help us see distant objects more clearly. In
a refracting telescope, light from a distant object is
collected and focused by a convex lens called the
objective lens.
0 A second lens, called the eyepiece lens, work as a
magnifying glass to enlarge the image.
Telescopes
0 A reflecting telescope uses a concave mirror to collect
rays of light from a distant object. The mirror is called
the primary or objective mirror.
0 It forms a real image, which is then magnified by the
eyepiece lens.
Telescopes
0 The lens in a refracting telescope and the mirror in a
reflecting telescope both act as a collector of light.
0 Reflecting and refracting telescopes must have a large
collector (either a lens or a mirror) in order to gather
as much light as possible from the distant object.
0 The collector then focuses the light into an image.
Telescopes
0 The farther the image is from the lens, the greater the
magnification. Similarly, the farther the image is from
the mirror in a reflecting telescope, the greater the
magnification.
0 For the greatest magnification, the telescope needs to
have as large a distance as possible between the
object being viewed (like a star or a planet) and its
image.
Binoculars
0 Binoculars are actually two reflecting telescopes
mounted side by side.
0 You can imagine how difficult it would be to hold up
two telescopes!
0 In binoculars, the telescopes are shortened by placing
glass blocks inside. These glass blocks, called prisms,
serve as plane mirrors.
0 Rather than travelling down the length of a telescope,
light in binoculars is reflected back and forth inside a
short tube.